It is conventional to grind synthetic resins and elastomers into powders of varying degrees of fineness. The degree of comminution attainable depends on the type of grinding apparatus utilized, the grinding conditions, and certain properties of the polymers employed.
It is also known that elastomer powders produced by grinding have a tendency to recombine into more or less large, coherent clusters or lumps, especially under temperature conditions such as those which occur during transportation or storage, for example. This presents an obstacle to ensiling, to conducting the dry-powder mixing technique conventionally used with pulverulent synthetic resins and also to utilizing the advantageous processing called powder elastomer technology.
To prevent this undesired caking in the conventional grinding process, according to the disclosure of DOS [German Unexamined Laid-Open application] No. 2,648,301 [=U.S. Pat. No. 4,157,790], it is necessary to add powdering agents (fillers) to the granulated elastomer. These are added in such large amounts that the powdered, pulverulent elastomer no longer exhibits the properties of the unpowdered elastomer during further processing, especially its mechanical properties. For this reason, the aforementioned DOS No. 2,648,301 describes and claims a process for the production of elastomer powders low in filler content in accordance with the grinding method, making it possible to make do with a content of powdering agent of less than 7% by weight. The multistage process comprises first converting the elastomer by mechanical preliminary comminution into large-surface and pore-rich particles; introducing the total amount of powdering agent through nozzles into the pneumatically conveyed stream of elastomer particles, so that the powdering agent is almost entirely adsorbed by the elastomer particles and the left-over surrounding powdering agent is conveyed to the grinding step with an extensively uniform distribution between the powdered elastomer particles; executing the grinding of the elastomer particles at a low temperature so that flowing of the elastomer is prevented and the large-area and recess-endowed structure of the pulverulent elastomer particles created by the grinding step remains preserved; effecting an adsorbing of any still unbound or freshly fed powdering agent by the elastomer powder in a pneumatic conveying zone following the grinding process; and cooling the thus-obtained powdered elastomer powder to such an extent that no warm flow of the elastomer occurs.
The results and requirements of such prior art processes leave much to be desired.